Time of flight ranging systems are commonly used in level measurement applications, and are referred to as level measurement systems. Level measurement systems determine the distance to a reflector, i.e. reflective surface, by measuring how long after transmission of energy pulses or waves, an echo is received. Such systems typically utilize ultrasonic pulses, pulse radar signals, or microwave signals.
Pulse radar and microwave-based level measurement systems are preferred in applications where the atmosphere in the container is subject to large temperature changes, high humidity, dust and other types of conditions which can affect propagation. To provide a sufficient receive response, a high gain antenna is typically used. High gain usually translates into a large antenna size.
Two types of antenna designs are typically found in microwave-based level measurement systems: rod antennas and horn antennas. Rod antennas have a narrow and elongated configuration and are suitable for containers having small opening/flange sizes and sufficient height for accommodating the antenna. Horn antennas, on the other hand, are wider and shorter than rod antennas. Horn antennas are typically used in installations with space limitations, for example, vessels or containers which are shallow.
The dielectric rod antennas used in microwave-based level measurement systems are intended for transmitting and receiving polarized microwaves with low crosspolarization in a large frequency bandwidth. Such antennas usually include a transition structure that introduces a propagating wave in a waveguide within the antenna assembly. The transition structure within the rod antenna assembly typically receives electromagnetic energy from a coaxial cable in which a TEM electromagnetic field is propagating. The energy propagates as a TE11 electromagnetic field within the waveguide and, at the end of the waveguide, is converted into an HE11 electromagnetic field. The HE11 electromagnetic field mode propagates within the rod antenna and is launched into free space as a TEM electromagnetic field. A difficulty that arises is that waveguides that are efficiently designed for propagating the fundamental TE11 mode often also propagate the upper TM01 mode. The TM01 mode is coupled from the transition structure into the rod antenna portion of the antenna assembly with poor efficiency and is mostly reflected back into the waveguide, resulting in a poor pulse echo profile.
Accordingly, it would be advantageous to have an improved dielectric rod antenna assembly having a transition structure that provides for pure TE11 propagation with low crosspolarization and maximum attenuation of upper modes. It would also be advantageous to have an improved dielectric rod antenna assembly with efficient coupling between the waveguide and the rod antenna.